In known apparatus of this type, the receiver is a "four-quadrant receiver" i.e. it is formed by four optically and electrically independent components whose sensitive surfaces or quadrants are juxtaposed along two mutually perpendicular axes in the focal plane of the lens means. This receiver has four electric outputs connected to an electric circuit which transforms the data delivered by the receiver into electric signals that are representative of the position of the image of the target relative to said perpendicular axes, whereby the signals are representative of the angular position of the target relative to the axis of the lens means, the orientation of this axis being known. In practice, the image is sufficiently large to cover at least part of all four quadrants and the signals delivered by the receiver are generated by comparing the light energy received by each quadrant.
The disadvantage of this apparatus is that it requires the four receiver components to be equally sensitive so as to obtain a suitably linear response from the electric circuit, even under severe environmental conditions such as those required for military equipment. In practice, this requirement is very difficult to meet and to try to do so as far as possible, low-sensitivity types of photoelectric receiver have had to be used.
Preferred embodiments of the present invention provide apparatus which is both robust and more accurate in determining the angular position of a target illuminated by light pulses.